Nov. 8, 2011 -- Babies who gain weight most quickly are at highest risk of obesity later in childhood, a Harvard study finds.

Infants who gain the most weight at age 1-6 months are most likely to be obese at ages 5 and 10 years. The risk goes up with the child's starting weight-for-length percentile obtained from standard growth charts, find Elsie M. Taveras, MD, MPH, and colleagues.

The researchers analyzed health records for 44,622 kids aged 1 month to 11 years from 1980 through 2008. They collected height and weight measurements made during the children's first 24 months, as well as their height and weight at ages 5 and 10.

Especially in their youngest years, children typically pass one or more of these percentile lines. That is, they may go from being in the 75th percentile of weight for length in one six-month period to being in the 90th percentile in the next, and in the 95th percentile in the six-month period after that.

But, "upward crossing of major weight-for-length percentiles in infancy, especially in the first six months of life, is associated with high rates of obesity at ages 5 and 10 years," Taveras and colleagues report in the November issue of Archives of Pediatrics and Adolescent Medicine.

Jumping upward across two or more of these percentile lines in six-month periods during the first two years is what puts kids at risk -- even very young children, Taveras and colleagues find.

Kids who passed two or more weight/height percentile lines by age 24 months had double the risk of being obese at age 5, and a 75% higher risk of being obese at age 10.

True, 43% of kids cross two or more percentile lines by age 6 months. Not all of them become obese.

But even kids who started out at lower weights were at risk of obesity. For example, among 6-month-olds who started out at the 25th percentile and then passed two or more percentile lines in the next six months, 7.4% were obese at age 5.

Taveras and colleagues note that percentile changes in weight for height are easy for a pediatrician to measure.

What is harder is to know what, if anything, should be done about it.

"It remains to be seen whether interventions based on such early identification result in improvements in child health," Taveras and colleagues note.